Question: I’m somewhat skeptical about the existence of the Oort Cloud. Some of the estimates I’ve heard indicate that it could stretch halfway to Proxima Centauri. Unless the interstellar medium is itself far less “empty” than predicted, it would seem unlikely that our star would have such a cosmic structure surrounding it that other stars lacked. Do we have any direct observational evidence that would confirm the existence of the Oort Cloud (evidence not explained by other theories)? Is it possible that the interstellar medium is simply less empty than predicted, and Oort-Cloud-like filler is common in interstellar space? – Tim
Answer: There is what one could call “indirect observational evidence for the Oort Cloud. It has been known since 1932 (first proposed by Ernst Opik, then updated by Jan Oort in 1950) that one needs a source for long-period comets that is beyond the orbit of Pluto. This source of long-period comets, which are gravitationally-bound to our Sun, cannot be interstellar. Also, we have seen Kuiper Belts (debris orbiting at distances from 30 to 50 AU) around other stars, and as the Oort Cloud is likely a continuation of the Kuiper Belt around our Sun, these properties appear to be common remnants of the star formation process. Note that our abilities to detect small bodies (a few to 10s of kilometers in diameter) in the Kuiper Belt and Oort Cloud regions are improving, using techniques such as background star occultation, making the ultimate characterization of the Oort Cloud inhabitants in the next few decades a good possibility.